Attaching one object to each hand of a Mixamo action
I may be wrong but in my opinion it will be much easier to do the opposite, aka make the hands the children of the shovel, because you can't make the shovel the child of 2 different bones.
You could make the shovel child of one hand (the hand that holds the back of the shovel) and make it track to the other hand (the one in front of the shovel), but you'll constantly have to rotate the hands so that they fit with the shovel angle, while if you do the opposite, aka the shovel controlling the hands, they will always twist correctly.
In that case, and more precisely, make the hand contollers (aka the targets of your arms IK constraints) the children of the shovel. You just need to give a Child Of constraint to both the 2 hand controllers with the shovel as Target: